1. Field of the Invention
This invention relates to new compositions comprising copolymers of tetrahydrofuran, ethylene oxide, and an additional cyclic ether.
2. Description of the Related Art
Homopolymers of tetrahydrofuran (THF, oxolane), i.e., polytetramethylene ether glycols, are well known for use as soft segments in polyurethanes. These homopolymers impart superior dynamic properties to polyurethane elastomers and fibers. They have very low glass transition temperatures but their crystalline melt temperatures are above room temperature. Thus, they are waxy solids at ambient temperatures and need to be kept at elevated temperatures to prevent solidification.
Copolymerization with a cyclic ether has been used to reduce the crystallinity of the polytetramethylene ether chains. This lowers the polymer melt temperature of the polyglycol and at the same time may improve certain dynamic properties of a polyurethane which contains such a copolymer as a soft segment. Among the comonomers used for this purpose is ethylene oxide, which can lower the copolymer melt temperature to below ambient, depending on the comonomer content. Use of copolymers of THF and ethylene oxide may also increase certain dynamic properties of polyurethanes, for example elongation at break, which for some end uses is desirable.
Copolymers of THF with ethylene oxide are well known in the art. Their preparation is described e.g. by Pruckmayr in U.S. Pat. No. 4,139,567 and U.S. Pat. No. 4,153,786. Such copolymers can be prepared by any of the known methods of cyclic ether polymerization, described for instance in “Polytetrahydrofuran” by P. Dreyfuss (Gordon & Breach, N.Y. 1982). Such polymerization methods include catalysis by strong proton or Lewis acids, by heteropoly acids, as well as by perfluorosulfonic acids or acid resins. In some instances it may be of advantage to use a polymerization promoter, such as a carboxylic acid anhydride, as described in U.S. Pat. No. 4,163,115. In these cases the primary polymer products are diesters, which need to be hydrolyzed in a subsequent step to obtain the desired polymeric glycols.
U.S. Pat. No. 5,684,179 to Dorai (Dorai) discloses the preparation of diesters of polytetramethylene ethers from the polymerization of THF with one or more comonomers. While Dorai includes 3-methyl THF, ethylene oxide, propylene oxide, etc., it does not describe a glycol copolymer of THF, ethylene oxide, and cyclic or substituted cyclic ethers.
Glycols formed as copolymers of THF and ethylene oxide offer advantages over homopolymer glycols in terms of physical properties. At ethylene oxide contents above 20 mole percent, the copolymer glycols are moderately viscous liquids at room temperature and have a lower viscosity than polytetrahydrofuran of the same molecular weight at temperatures above the melting point of polytetrahydrofuran. Certain physical properties of the polyurethanes prepared from THF copolymers surpass the properties of those polyurethanes prepared from THF homopolymers.
However, there are certain disadvantages connected with the use of ethylene oxide (EO) in these copolymers. EO is quite hydrophilic and can increase the water sensitivity of the corresponding polyurethanes when used in the required concentrations.